The present invention relates generally to Electrostatic Field measuring devices and, more particularly, to sensor configurations and signal processing methods that provide enhancements in the bandwidth and suppression of unwanted signals or noise.
There is a long history of people studying atmospheric electricity. Storm prediction and analysis of electric fields and discharges (lightning) associated with storm clouds have necessitated the creation of several types of geophysical sensors and instruments. The field mill is a geophysical sensor that is designed to measure the electric fields below and near storm clouds.
Chalmers, J. Alan, Atmospheric Electricity, 1957, New York Pergamon Press, pp. 212-217 provides a description of the distribution of charges in clouds and the resulting field observed at the ground level. The most common storm cloud has a positive charge distribution in the top of the cloud and a negative charge distribution in the bottom of the cloud. This is called a positive cloud and is most common in the regions studied, although other charge distributions do exist in clouds. Monitoring the electric field at ground level as a positive storm cloud approaches would show a positive field if the cloud is at some distance and a negative field as the cloud passes overhead. As the cloud recedes, a positive field would again be observed. This can be predicted by considering the geometry of the cloud charge distribution in relation to the observation point. In clear weather, the typical field at ground level has been measured to be about 100 to 300 volts/meter, whereas the field associated with storm clouds is typically in the thousands of volts/meter. These field measurements are considered to be quasi-static since they are either constant or vary at a low rate of change. In order to measure these quasi-static fields, the field mill was developed. Field levels associated with lightning exhibit a rate of change that is higher than can be tracked by the typical field mill.
Chalmers, supra, also cited a study in England by Wormell involving the use of an inverted test plate connected to an electrometer. The test plate was shielded from above to protect it from rain. As the test plate was covered and uncovered, the electrometer registered the field due to the cloud charge in the region. This prior art study could be considered a manual version of the field mill developed later. Wormell stated that he could only measure the quasi-static or stable fields before the lightning flashes, not the rapidly varying values that occurred after the flashes. This limitation was due to the limited bandwidth of the instrument.
Kessler, Instruments and Techniques for Thunderstorm Observation and Analysis, vol. 3, 1983, Normand: University of Oklahoma Press, pp. 89-102, in the chapter authored by Edward T. Pierce, it is stated that although many designs for field mills exist, all operate on the same basic principles. The principles Pierce describes come from basic electrostatic theory and can be summarized as follows. If a conducting plate having a surface area A is exposed to an electrostatic field of magnitude E in a medium such as a vacuum or air (free space) that has a permittivity e0, a surface charge of e0*E*A will be generated on the surface of the plate. If the conducting plate is shielded, this surface charge will be effectively zero. Thus, if the conducting plate is repetitively exposed to and shielded from the electrostatic field, the resulting charge waveform will be an alternating one. This charge waveform can be further processed to extract the magnitude of the field E.
The typical field mill cited in the Kessler reference consists of a set of conducting plates, insulated and alternately shielded from and exposed to the electrostatic field by a rotor having open areas and solid areas. This structure produces an exposed plate area that varies as a triangular function of time and in this example has a frequency of 100 Hz. After this triangular signal is demodulated by a signal that is synchronized with the rotor, the triangular signal is integrated over a time constant that often exceeds 0.1 second. This integration (low pass filtering), along with the variation in exposed plate area over time rendered this prior art field mill unsuitable for studying electrostatic fields that change rapidly, such as those related to lightning discharges. The need for additional instrumentation in such measurement applications is recognized by the prior artisan Pierce.
According to the Kessler reference, cited above, it is apparent that prior art investigator Pierce was aware of the inherent bandwidth limitations of the typical field mill. His statement that the time constant exceeds 0.1 second is equivalent to saying that the upper bandwidth limit of a field mill was less than 1.6 Hz. Since a field mill can measure quasi-static fields, this is equivalent to saying that the lower field mill bandwidth limit extends down to DC. Combining these facts gives a specification of the field mill bandwidth of DC to 1.6 Hz.
Studies of atmospheric electricity and other field measurement activities require the capability of measuring the changes in field level that occur at a rate faster than a typical field mill can measure. This requires an instrument with an upper bandwidth limit greater than that of the field mill.
In the Kessler reference, Pierce describes some of the additional instrumentation that could be used to measure rapid changes in the magnitude of electrostatic fields that extend beyond the capability of the prior art field mill. This additional instrumentation takes the form of a slow antenna and a fast antenna. One form of the slow and fast antennae, preferred due to its ease of calibration, is a flat conducting plate of area A, insulated from, but flush with the earth""s surface. In accordance with the basic electrostatic theory of field mill operation summarized above, the resulting charge on the flat conducting plate due to the varying field E(t) above the plate is e0*E(t)*k The inherent structure of these antennae and the subsequent amplification can be modeled as a source with a shunting resistance R and capacitance C such that the output voltage will be proportional to e0*E(t)*A/C, with a decay time constant of R*C. In the interest of measurement accuracy, it is prudent that this decay time constant be at least ten times the duration of the change in the electrostatic field to be measured. For a slow antenna, with a typical 10-second decay time constant, the duration is limited to 1 second. For a fast antenna, with a typical 100-microsecond decay time constant, the duration is limited to 10 microseconds.
The time constants that Pierce described can be also stated as the lower bandwidth limit of these types of antennas. A slow antenna with a time constant of 10 s has a lower bandwidth limit of 0.016 Hz, and a fast antenna with a time constant of 100 us has a lower bandwidth limit of 15.9 KHz. Neither of these types of antennas can measure the quasi-static field such as the field mill does, since their lower bandwidth limit does not extend down to DC. This is not possible due to the inherent resistance in the insulator holding the antenna, as well as that of the amplifier circuit. The upper bandwidths of these antennas are limited by the amplifier circuit and the need to discharge the previous measurement before a second measurement is performed. This is why a fast antenna with a low time constant is used for measuring the multiple rapid field changes associated with a return lightning stroke rather than using a slow antenna for both measurements. If this were not done, the amplifier would saturate and thereby be rendered useless for subsequent measurements until returning to equilibrium.
It may be seen that it is necessary to utilize three prior art instruments to measure over the range of bandwidths required to study the field changes associated with storm clouds and the field changes caused by lightning within those storm clouds.
It has been stated that the flat plate field mill, slow antenna, or fast antenna, mounted flush with the ground is the most convenient form factor for calibration. In this case a screen at a height X over the sensor can be charged to a voltage V. The resulting calibration field is simply V/X. It is also prudent to make the dimensions of the screen much larger than those of the sensor to maintain a constant field over the sensor. At the edges of the screen a fringing field effect will cause the field to be at a different level from the desired calibration field.
Although other form factor field mills and field measuring devices are known in the prior art, the flat plate field mill is often used due to its ease of calibration and correlation to the mathematical basis. The other form factor field mills may detect a particular field at a higher or lower level than the standard flat plate field mill due to their geometric factors. A standard flat plate field mill can be used to simultaneously measure a field and determine a gain change required for the alternate form factor field mill such that calibration can be accomplished.
In another chapter of the Kessler reference, W. David Rust and Donald R. MacGorman describe more details of a typical rotating-vane field mill of the flat sensor plate form factor. The field mill they describe has four sensor plates, each occupying 90 degrees of the mechanical rotation of the rotor. The grounded rotor has two 90-degree openings opposed at 180 degrees. The opposing two sensor plates are connected together to form two sensor signals out of phase. The sensor signals (charge) are converted to voltages and are amplified, demodulated, and filtered to obtain a voltage proportional to the field. The example chosen uses an inverting charge amplifier that maintains the sensor plates at ground, an arrangement that has advantages over the non-inverting charge amplifier. Rust and MacGorman state that the ideal waveforms from the charge amplifiers should be triangle waves, but due to the fringing field at the edge of the sensor plates and rotor openings they appear more as sine waves. After demodulation, based on a signal generated by a replica of the rotor disk with openings and an optical interrupter, the signal is filtered to extract the average voltage that is proportional to the field intensity. The resulting upper bandwidth limit is less than 10 Hz. Although this is higher than was achieved by the previous example, it is still unusable to track the rapid changing field conditions associated with storms. This indicates that additional slow and fast antennas are still required to measure the field over the required bandwidth.
The first amplifier stage that processes the sensor signals is critical to the accuracy and performance of the field mill, a slow antenna, or a fist antenna. Besides exhibiting a very high input impedance, low leakage currents, and low noise, the chosen amplifier configuration provides various advantages and disadvantages. The two basic types of amplifier configurations are non-inverting and inverting. In the non-inverting amplifier configuration, the sensor plates have a voltage on them with respect to ground. This requires special insulation and guarding techniques to prevent errors due to leakage currents. Guarding requires driving a shield with the output of the amplifier and can cause a degradation of the upper bandwidth limit. An example of this amplifier is shown in U.S. Pat. No. 3,846,700 to Sasaki et al.
The inverting amplifier configurations are of two types. One is the charge amplifier configuration in which a capacitor element is used in the feedback path such that the output voltage waveform is proportional to the charge on the sensor plates. As the rotor exposes the sensor plate area, the resulting output voltage is ideally a triangular waveform, but is practically observed as being sinusoidal. This implementation is pleasing from a mathematical standpoint, but due to its shape, requires additional filtering to extract the average voltage level. A high impedance resistive element is typically added to the feedback path to provide control of the DC operating point of the amplifier, but this does not modify the basic concept of the charge amplifier, as described by Rust and MacGorman.
The other type of inverting amplifier configuration utilizes a fully resistive feedback element, which defines the output voltage as being proportional to the input current from the sensor plate. Since current from the sensor plate is representative of the rate of change of charge (and area in this case), the resulting ideal waveform is a square wave. In practice, the waveform is more trapezoidal in shape due to the fringing effects at the edges of the sensor plates and at the rotor openings. This amplifier configuration requires less filtering but is more sensitive to rotor speed than the charge amplifier. Wider bandwidths could be achieved with this amplifier over the other amplifier configurations if the effects of fringing were removed. In both inverting amplifier configurations, the sensor plate is held close to ground potential so special guarding and shielding is not required, and the resulting additional bandwidth limitation of the non-inverting configuration is removed.
U.S. Pat. No. 4,370,616 to Williams recognizes some of the advantages of this xe2x80x9clow impedancexe2x80x9d amplifier configuration used in a non-contact electrostatic voltmeter that is a type of field measuring device. Since the sensor plate is held at ground, no expensive or power inefficient guarding drivers are required, and it is more immune to humidity and contamination on the insulation supporting the sensor plate. This reference also teaches the desirability of having an increased rate of response or bandwidth when such a device is used for making measurements in production applications.
The resulting trapezoidal voltage waveform from the low xe2x80x9cimpedancexe2x80x9d current-to-voltage converter used in a field mill application was not mentioned since the particular device presented uses means other than a grounded rotor, or shutter, to vary the coupling to the field or voltage being measured.
In Chang et al., Handbook of Electronic Processes, 1995, New York: M. Dekker, pp. 228-235, the chapter written by Mark N. Horenstein describes several field measuring and non-contact electrostatic voltmeters. It can be deduced by the mathematics presented, explaining the operation of the field mill, that the xe2x80x9clow impedancexe2x80x9d inverting current-to-voltage amplifier stage was used in this device. The resulting waveform was shown as a square wave (ignoring fringing field effects). This waveform was sampled when the rotor or shutter opening was centered over the sensor plate. Although this method ignores the error associated with the fringing fields at other rotor locations, this sampling at a low multiple of the rotor revolution rate limits the upper bandwidth limit of this device. Another way to view this limitation is to consider that the device is unaware of any field changes in between samples such that rapidly varying fields cannot be measured. Additional filtering of the sampled voltage waveform is also required, which further limits the upper bandwidth limit.
It has been stated that the field mill in a flat plate format is the most general case of the implementation of the instrument and is the most straightforward to calibrate. Many other formats or configurations have been devised to match the measurement application. All of these configurations operate in the same basic method and could all benefit from an increase in upper bandwidth limit.
The flat form factor field mill has a set of sensor plates in a common plane and has the shape of a radial segmented disk. The rotor or shutter is also a disk that is the same size or larger diameter than the sensor array disk. It contains openings that periodically expose the sensor plates below it. Since it is grounded, it shields the corresponding sensor plate below it when the solid portion of the disk is above it. Fields are sensed in an axial direction with respect to the axis of rotation. Examples of this standard configuration can be found in the Kessler and Chang et al. references.
If the flat form factor field mill is inverted such that the sensing direction is toward the earth and elevated a distance above the earth, the effects of precipitation on the sensor plates can be eliminated. Since the elevated field mill assembly is still grounded, some of the lines of the atmospheric field bend around and upward into the sensor plate assembly. The gain of this geometric form factor is different than that of the flush mounted flat form factor field mill. By comparing the readings of these two configurations at the same time at various field strengths and polarities, the inverted form factor field mill may be calibrated. An example of this form factor is described in U.S. Pat. No. 4,424,481 to Larocke et al.
If the perforated rotor and the enclosed segmented sensor plate array are formed in a cylindrical form factor, along with grounded shields at the ends of the rotating cylinder, a field mill is produced that has applications in moving vehicles such as aircraft. In a variation of this cylindrical field mill, the grounded shutter and the insulated sensor plate are opposing hemispheres of the same cylinder. An example of this form factor is shown in the Kessler reference, mounted on the nose of an aircraft. The field is sensed in the radial direction. A particular direction can be selected by the phase of the demodulation signals. In this manner two orthogonal fields can be measured with the same field mill.
Another form factor field mill is formed by shaping the flat form factor field mill into a cone shape. This is done to shed precipitation from the spinning rotor and underlying conical sensor plate array. This allows the field to be sensed in an upright direction and yet have some resistance to the effects of the precipitation. An example of this form factor is shown in U.S. Pat. No. 4,055,798 to Kato.
The shutter mechanism can also be formed with a tuning fork structure powered to maintain the mechanical oscillation. The sensor plates would be shaped in order to be covered by the grounded tuning fork member during part of the oscillation cycle and uncovered during another part of the cycle. Although no rotary shutter is incorporated, the same basic method employed in the rotary field mills is used to measure the field. An example of this form factor is shown in the Chang reference.
In all of the prior art field mill examples described above, the variable coupling of the sensor plates to the field was done with mechanical, moving, grounded shields or shutters. In some applications, the coupling of the sensor plate to the field can be accomplished by electronic means. An electrically modulated grid between the field to be measured and the sensor plate has been described in U.S. Pat. No. 3,812,419 to Kaunzinger et al. A solid state electronic means for intermittently grounding the sensor plate exposed to the field has been described in U.S. Pat. 4,642,559 to Slough. Although these electronic means have some advantages in the areas of power and ruggedness, they may not be suitable in applications requiring high accuracy, sensitivity, and dynamic range, such as is encountered in the measurement of atmospheric electricity. In that application, mechanical shutter field mills still dominate.
A non-contacting voltmeter is another variation on the field mill sensor, as discussed in the previously cited Chang reference, for use in electrostatic laboratories and production lines. Feedback-null surface potential monitors represent one class of non-contacting voltmeter. Such instruments measure the field at a distance from a charged surface. If the charged surface is a conducting surface, its voltage level can be deduced by measuring the field at the sensor.
As discussed above, the upper bandwidth limit of prior art field mills is not high enough to provide all of the field measurements desired for studying storm cloud systems. Rust, David W., Utilization of a Mobile Laboratory for Storm Electricity Measurements, Journal of Geophysical Research, vol. 94, no. D11, Sep. 30, 1989, pp. 13,305-13,307 describes a mobile laboratory used to make these types of storm electricity measurements. In this case, a field mill and three flat-plate charge antennae (two slow antennae and one fast antenna) are used to collect the desired bandwidth of data representing the field characteristics below tornado producing thunderstorms.
It can be seen that it would be desirable to provide a wide bandwidth field mill having a bandwidth lower limit from DC to an upper bandwidth limit in the range of frequencies that the fast antenna typically measures. This would allow a single instrument to record all of the data typically requiring three or four instruments to capture. The correlation of the various frequency bands could be completely preserved in a wide bandwidth field mill, thus eliminating the additional timing reference data signals, increased data storage means, and careful processing that are presently required in order to combine the field information from multiple prior art instruments. The higher bandwidth of field mill-like sensors is also desirable in many other applications since it would allow for improved measurements, faster production, and improved research. It would also be desirable to provide a field mill having the capability of rejecting undesirable disturbances and field transients, such that the measurement accurately describes the actual field level over the course of the measurement.
Briefly stated, the present invention involves a novel sensor plate configuration that provides much more information to be processed as compared to the prior art. The present sensor plate configuration provides more continuous exposure to the field being measured and, together with differential signal processing, provides an added degree of suppression of field rate of change transients over prior art low pass filtering. The present invention reduces this filtering need and therefore provides a wider bandwidth capability. Another aspect of the present invention involves generating a new configuration of demodulation signals required to process the signals from the sensor plates. The signals from the sensor plate configuration are processed by two differential signal paths that provide suppression of unwanted signals and increased signal bandwidth and information content to a two-stage demodulator. The two-stage demodulator and a novel timing signal generation configuration provides accurate demodulation of the two differential sensor signal paths into one differential field magnitude and polarity path. This combined data path is further processed to remove the common mode noise and, in one embodiment, to provide a buffered differential signal for the transmission path to a control and display unit. The differential sensor signal paths include AC amplifiers having multiple selectable gains that allow for optimizing the measurement range of the instrument to the field magnitude range of interest. The present invention can be applied to any of the configurations described in the prior art examples presented above to provide significant advantages.